There are previously known many different variants of propulsion devices of the above-mentioned type. For example, the U.S. Pat. Nos. 2,021,815 and 2,367,765 show propulsion devices for a watercraft wherein vertical movement of the craft is transferred to the plate-like wing or foil through a carrier structure for the wing, the wing being pivotable about a shaft attached to the carrier structure. The excursion angle of the wing is limited by mechanical stops placed on the carrier structure.
The devices according to said U.S. patents are not capable of utilizing small vertical movements of the craft, since the wing must have such large excursions as to bear against the stop before propulsion force can be obtained. For really large vertical movements the wing will stall, and great eddy formations around the wing will cause losses and little efficient propulsion.
U.S. Pat. No. 4,332,571 shows an improved construction in relation to the two above-mentioned U.S. patents. In the wave motor according to this patent there are used steel springs or hydraulic/pneumatic cylinders with e.g. gas pressure in order to exert forces trying to return the wing to its neutral horizontal position when making excursions from its neutral position in case of relative movement between water and wing. In this construction, also small vertical movements can be utilized, as well as the large movements which might occur, since the wing can operate with all excursion angles up to .+-.90.degree..
U.S. Pat. No. 4,371,347 shows a propulsion device having a wing driven by waves and/or motor force, and wherein a carrier structure for the wing is vertically movable in relation to the watercraft. The carrier structure has a neutral central position, and springs or hydraulics try to bring it back to this position if it has been brought out from this position because of a large vertical movement of the craft. The wing is mounted at the lower end of the carrier structure and is provided with stops for limitation of its angular excursion. This device primarily is intended to provide propulsion by means of wave energy, but it may also be utilized to provide propulsion by means of a motor in that the carrier structure and the associated wing are driven up and down by means of motor force.
A common weakness of the known propulsion devices of the type in question is that they lack the possibility of reversing the propulsion direction of the driven watercraft. This is particularly important if the craft gets its propulsion from a wing (or wings) and wave energy only. However, a topical field of use is also the utilization of wave energy as a supplementary propulsion for boats and ships driven by a conventional motor-driven screw propeller. Less motor power is then necessary in order to maintain the same speed, and thereby one economizes on fuel. A watercraft which is equipped with wings according to said known principles, will have problems when it is going to reverse with its screw propeller. When the water flow starts coming in from behind adjacent to said wings, these will swing out and cause a strong braking of the movement of the craft backwards. The wings will also be subjected to extreme forces, and damage of the wings and their carrier structures is a possibility.